In overhead transmission lines or overhead lines (OHL) auto-reclosing protection may be utilized. A fault protection or detection unit may be employed for monitoring one or several transmission lines of a power transmission network. In the event of a fault on a transmission line, the fault protection unit trips circuit breakers located at each end of the transmission line, for the purpose of disconnecting the transmission line having a fault from the rest of the power transmission network. Thereby, the risk of propagation of the fault in the power transmission network may be mitigated. Subsequently, depending on the type of fault, the circuit breakers may be closed in order to restore power transmission via the transmission line that was previously disconnected from the rest of the power transmission network. This may be referred to as auto-reclosing.
In OHLs most of the faults are typically temporary or transient faults, e.g., due to lightning strokes, and an auto-reclosing protection scheme may therefore be applied. On the other hand, in transmission lines based solely on cables, virtually all faults are permanent faults, and auto-reclosing is therefore not applied. Permanent faults may occur also in OHLs. In case of a permanent fault, the fault typically has to be cleared or removed by maintenance personnel.
Considering as an example a multi-terminal voltage source converter (VSC) based High Voltage Direct Current (HVDC) power system, faults on a DC cable or DC OHL are typically isolated from the rest of the power system or another part of the power system by temporarily shutting down the DC line, or temporarily taking the DC line out of operation, using DC circuit breakers.
For an OHL, the DC line is put back into operation after some time has passed to allow for the fault to be cleared, i.e. to allow for the fault arc current to be extinguished, in case it is a temporary fault. However, if the fault is a permanent fault, the DC line will be shut down again after reclosing it. In that case, the DC line will be shut down for a relatively long period to allow for maintenance personnel to locate and clear the fault. Events leading to DC line faults could happen both during normal operation when the DC line is energized, or when the DC line is not energized. In case the DC line is not energized when the fault occurs, the fault may only manifest itself when the DC line is attempted to be brought back into service or operation.
For faults in DC OHLs, DC circuit breakers are typically designed to auto-reclose after some time period, with the length of time period being based on the time it takes for the fault arc current to be extinguished. In case of temporary faults, the DC circuit breakers will first be tripped, and the DC line will then be restored back to normal operation following successful auto reclosing. In case of permanent faults however, the DC circuit breakers need to be tripped again after auto reclosing since there may still be a fault present in the DC line. It would therefore desirable for a DC line protection system or mechanism to be able to detect if the fault is still present after auto reclosing has been carried out. It would also be desirable that such detection could be carried out relatively quickly, with a speed comparable to a primary protection system, due to the high rate of rise in currents occurring in e.g. VSC based HVDC power transmission systems, or grids. Similarly, it would also be desirable to be able to detect if a fault in a DC line is present after having switched DC circuit breakers in order to energize an open DC line. In energizing an open DC line, a protection mechanism should be in place to ensure that a fault is detected if there is a fault present in the DC line.